We propose to study prevention and treatment of acute graft- versus-host disease (GVHD). We will attempt to improve prevention of GVHD and evaluate, in a prospective randomized trial, whether the addition of prednisone during the first five weeks after marrow grafting will reduce the incidence of acute graft-versus-host disease in patients on prophylaxis with methotrexate/cyclosporine. We also will shorten the overall administration of immunosuppression to 60 days and compare this regimen with the customary 180 days; this is done in the hope of decreasing the risks associated with prolonged immunosuppressive therapy. We also plan to compare the effectiveness of antithymocyte serum to that of prednisone in the treatment of established GVHD In patients on cyclosporine prophylaxis. We have a continued interest in exploring various monoclonal antibodies against T-cells for their efficacy in treating established GVHD. Finally, we would like to evaluate the usefulness of an anti-IL-2 receptor antibody first in treatment and then in prevention of acute GVHD.